Apex domains cannot have CNAME records.
Question and response
Question
dig bf1.yahoo.com. NS
Answer
bf1.yahoo.com. 5 NS ns1.yahoo.com.
bf1.yahoo.com. 5 NS ns4.yahoo.com.
bf1.yahoo.com. 5 NS ns2.yahoo.com.
bf1.yahoo.com. 5 NS ns3.yahoo.com.
bf1.yahoo.com. 5 NS ns5.yahoo.com.
Authority
The response has no authority records.
Additional
ns1.yahoo.com. 5 A 68.180.131.16
ns2.yahoo.com. 5 A 68.142.255.16
ns3.yahoo.com. 5 A 27.123.42.42
ns4.yahoo.com. 5 A 98.138.11.157
ns5.yahoo.com. 5 A 202.165.97.53
ns1.yahoo.com. 5 AAAA 2001:4998:1b0:0:0:7961:686f:6f21
ns2.yahoo.com. 5 AAAA 2001:4998:1c0:0:0:7961:686f:6f21
ns3.yahoo.com. 5 AAAA 2406:8600:f03f:1f8:0:0:0:1003
ns5.yahoo.com. 5 AAAA 2406:2000:1d0:0:0:7961:686f:6f21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0